EP0216694B1 - Apparatus for real-time control of total penetration welding, in particular for seams inaccessible to direct observation - Google Patents

Description

The present invention relates to a device for real-time control of a welding with total penetration and relates more particularly to the control of the welding of a joint inaccessible to direct observation. The device of the invention is particularly suitable for controlling welding on a practically closed part or for welding two tubes of great length and small diameter.

The assembly by total penetration welding leads to the back of the weld to the formation of a luminous molten bath, in the case of TIG or MIG welding for example, or to the formation of a luminous plasma made up of metallic vapors. more or less ionized, in the case of electron beam or laser beam welding, possibly accompanied by fine droplets of molten metal. The observation of these phenomena during welding makes it possible to guarantee that the molten zone has affected the entire thickness of the parts to be assembled.

A device for real-time control of a total penetration welding comprises, in known manner, an optoelectronic means for detecting the light intensity on the back of the weld bead at the point of the welding point, a means for processing the electrical signal delivered by said detection means, and signaling means.

A detection means is known which consists of a housing containing an optoelectronic detector and an optical processing means. It is a bulky detection means which can only be used when the back of the weld bead is easily accessible to observation.

There is also known a detection means consisting of a set of photodiodes and a detection means consisting of a set of optical fibers connected to a photodiode. In these two known detection means, the set of photodiodes and the set of optical fibers are arranged so as to detect a light signal on a circumference. In these known detection means, each photodiode, or each optical fiber, constitutes a substantially omnidirectional detector. The number of photodiodes, or optical fibers, necessary to observe the entire weld bead is large. These detection means are therefore bulky and cannot be used for the control of a full penetration welding of a joint which is difficult to access for direct observation.

Another prior art is constituted by application FR-A-2 560 696 in which a device for controlling the welding of two metal parts is described. The device comprises an energy source for carrying out the welding and a means for detecting the light radiated by the vapor released at the welding point. This detection means, which is capable of comprising an optoelectronic detector as well as a bundle of optical fibers for transmitting to said detector the light intensity radiated at the weld, is placed on the same side as the energy source with respect to the parts to be welded so as to determine the depth of the weld from light radiation.

This document does not disclose the possibility of carrying out the control of the reverse side of a weld bead.

The invention aims to allow real time control of a full penetration welding in the case of a joint inaccessible or difficult to access for direct observation. This goal is essentially achieved by the use of a means of detection comprising an optoelectronic detector and a single optical fiber, the end of this optical fiber being cut into a cone so as to collect a light signal coming from any point of the weld bead.

Specifically, the subject of the invention is a device for real time control of a full penetration welding by detection of the light intensity present on the back of the weld bead at the time of welding, said device being particularly suitable checking the back of a joint inaccessible to direct observation, said device comprising an optoelectronic means for detecting said light intensity present on the back of the weld bead at the time of welding, a means for processing the electrical signal delivered by said detection means and signaling means, said device being characterized in that said optoelectronic detection means comprises an optoelectronic detector and a single optical fiber, said fiber comprising a first end connected to the optoelectronic detector and a second end cut in cone-shaped, said second end cone-shaped opening angle different from 180 ° being intended to be held stationary in a predetermined position facing, but set back, from the back of the weld bead to be formed on the parts to be welded, so as to collect the light intensity present on this reverse side of weld bead at the time of welding while being outside the range of thermal or mechanical aggressions due to welding, said opening angle of said end cone of the optical fiber being chosen such that the resulting field of vision for said end is able, when the latter is held stationary in said predetermined position, to grasp the entire joint defined by the parts to be welded.

According to a preferred embodiment, the angle of the end cone of the optical fiber is such that the entire weld bead is in the field of vision of the optoelectronic detector, when the optical fiber is held stationary. Preferably, the opening angle of this cone is between 25 ° and 45 °.

According to a preferred embodiment, the optical fiber is, at least partially, contained in a sheath. This is advantageously made of a ductile material which then simultaneously constitutes a protection and a mechanical support for the optical fiber.

Preferably, the processing means is adapted to deliver to the welding means a signal for controlling the intensity of the welding beam as a function of the intensity of the detected optical signal. This makes it possible to control the power of the welding beam and to ensure total penetration of the welding.

• la figure 1 repésente schématiquement un mode de réalisation du dispositif de contrôle de l'invention,
• la figure 2 illustre un mode de réalisation du moyen de détection du dispositif de l'invention,
• les figures 3a et 3b représentent respectivement un premier mode de réalisation de l'extrémité de la fibre optique et la caractéristique de transmission optique associée,
• les figures 4a et 4b représentent respectivement un second mode de réalisation de l'extrémité de la fibre optique et sa caractéristique de transmission optique associée, et
• les figures 5a et 5b représentent respectivement l'intensité du faisceau de soudage dans le cas d'un soudage par faisceau d'impulsions et l'intensité du signal optique relevé à l'envers de la soudure en fonction de ce faisceau d'impulsions.

The characteristics and advantages of the invention will emerge more clearly from the description which follows, given by way of illustration but not limitation, with reference to the appended drawings, in which:

• FIG. 1 schematically represents an embodiment of the control device of the invention,
• FIG. 2 illustrates an embodiment of the detection means of the device of the invention,
• FIGS. 3a and 3b respectively represent a first embodiment of the end of the optical fiber and the associated optical transmission characteristic,
• FIGS. 4a and 4b respectively represent a second embodiment of the end of the optical fiber and its associated optical transmission characteristic, and
• FIGS. 5a and 5b respectively represent the intensity of the welding beam in the case of a pulse beam welding and the intensity of the optical signal detected on the reverse side of the weld as a function of this pulse beam.

Schematically shown in Figure 1 a welding system provided with a control device according to the invention. By way of example, the case of welding, by a pulsed laser beam, of a high-performance pressure device has been shown. However, it is clear that the control device of the invention is not linked to the nature of the welding means but can on the contrary be associated with any known welding means, in particular electron beam welding, TIG welding, MIG or MAG.

The device to be welded consists of a bottom 2 and a ferrule 4. These two parts have the general shape of a hollow cylinder provided with a bottom. The ferrule 4 is provided with a small opening 6. The assembly of the two parts to be welded forms a closed hollow cylinder.

In the case of laser beam welding, the two parts to be welded are arranged on a support 8 disposed under an inert atmosphere in a welding enclosure 10. The laser beam 12 delivered by the welding means 14, outside the enclosure welding, enters the welding chamber through a window 16. The welding means is fixed. Welding on the entire joint between the two parts is obtained by rotation of these parts around a vertical axis. This rotation can be obtained by a turntable 18 constituting the upper part of the support 8.

The welding control device comprises an optoelectronic detection means 20, a processing means 22 and a signaling means 24. The detection means 20 is disposed in the welding enclosure 10. It is connected to the processing means 22 by an electrical link 26 passing through the wall of the welding enclosure 10. The processing means 22 may comprise an amplifier 28 followed by a servo means 30 intended to control, by a channel 32, the power of the beam of the means of welding 14 as a function of the optical signal detected on the back of the joint. The signaling means 24 can comprise a display means 34 (plotter, screen or other) and a recording means 36. These means 34 and 36 simultaneously receive a first electrical signal corresponding to the detected optical signal and a second corresponding electrical signal the intensity of the welding beam. This second signal is delivered by the welding means to the processing means 22 on a channel 38.

According to the invention, the detection means 20 comprises an optoelectronic detector 40 and a single optical fiber 42. This fiber is preferably protected from mechanical attack (projections, metallic vapors, droplets of liquid metal or the like) by a sheath. Advantageously, this sheath is made of a ductile material, for example stainless steel or copper. This allows the optical fiber to be shaped so that it follows a well-defined path and that it remains in the optimal position without any special fixing device.

The optical detection means of the invention thus has the advantages of a very small footprint and very great flexibility allowing it to reach regions inaccessible, or very difficult to access, to direct observation.

According to the invention, the end of the optical fiber collecting the light signal on the reverse of the joint is cut into a cone shape. The angle of this cone defines the opening of the field of vision 43 and makes it possible to arrange the fiber out of the range of mechanical or thermal aggressions due to welding, which can damage the end of the fiber or simply disturb the measurement.

The length of the fiber is provided sufficient to distance the optoelectronic detector 40 from the welding area so that it does not interfere with the execution of the weld and that its operation is not disturbed by the environment (electric or magnetic field, heating, welding atmosphere or other).

FIG. 2 shows a longitudinal section of the optical detection means. The optoelectronic detector 40, consisting for example of a photodiode, is protected by an insulator 44 and disposed in a cylindrical support 46. Its sensitive optical part is disposed opposite one end of the optical fiber 42. The latter is protected by a sheath 48 which is positioned in its upper part, by a flange 50. This upper part is contained in a support 52. The end 56 of the optical fiber collecting the optical signal is cut into a cone shape. The field of vision 43 of the optical detection means depends on the opening of this cone.

Preferably, the support 46 of the optoelectronic detector and the support 52 of the optical fiber are provided with hooking means 58 which cooperate. This design makes it possible to make the optical fiber interchangeable and thus to adapt the optical detection means to the geometry of the weld to be checked.

In Figure 1, there is shown the case where the observation of the underside of the weld bead is difficult because the welded part constitutes a closed volume. The control device of the invention can also be used advantageously, for example, in the case of the welding of two tubes of great lengths and small diameters. Direct observation of the back of the weld bead, which is not possible with the optical detection means according to the prior art, does not present any difficulty with the optical detection means of the invention. In all cases, the end of the optical fiber is defined so that its field of vision covers the entire back of the weld bead. The opening angle of the end cone of the optical fiber also makes it possible to adapt the field of vision according to the position and the diameter of the weld.

Figures 3a and 4a illustrate two different shapes of the end of the optical fiber. Figures 3b and 4b illustrate the transmission power corresponding to each of these fibers as a function of the angle of incidence of a light signal.

In FIG. 3a, the opening of the end cone 56 of the optical fiber 42 is 40 °. The graph in FIG. 3b, representing the light intensity transmitted by the optical fiber as a function of the angle of incidence ϑ of a light signal, shows that the field of vision 43 is between the incidences and approximately 45. Given the necessary distance from the end of the optical fiber to the welding point, to avoid mechanical attack, this field of vision is suitable for controlling a welding operation on a circular joint of approximately 200mm in diameter.

In FIG. 4a, the opening of the end cone 56 is 30 °. The curve in FIG. 4b shows that then the field of vision 43 is between the incidences of around 50 to 60 °. Such a field of vision is particularly suitable for checking a weld on a joint having a diameter of the order of 100 mm.

The energy E is shown in FIG. 5a as a function of time t of the welding beam obtained by a YAG laser (aluminum garnet and yttrium semiconductor). It is a point-to-point welding, the welding being constituted by a succession of points with overlap, which ensures a continuous welding.

When the welding is at full penetration, there is, for each pulse of the laser beam, appearance on the reverse side of the welding of a plume of plasma and a corresponding light signal. The presence or absence of this light signal detected by means of the optoelectronic sensor makes it possible to assess the state of the weld on the reverse side of the weld bead.

FIG. 5b shows the intensity of the detected light signal, in correspondence with the intensity of the laser beam.

The delay between the start of the laser pulse signal and the start of the light signal on the reverse side of the weld is indicative of the quality of the weld penetration. For the pulse 60 of the laser beam, the delay ΔT₁ is low: there is over-penetration. For pulse 62, no light signal appears behind the weld bead: there is under-penetration. For pulse 64, the delay ΔT₂ indicates normal penetration. Finally, for pulse 66, the delay ΔT₃ is practically equal to the duration ΔT of the laser pulse: total penetration has just been reached.

The timing diagrams of FIGS. 5a and 5b make it possible to assess the quality of the penetration of the weld, to locate any lack of penetration and their coordinates relative to the starting reference position of the weld. They also make it possible to decide on the possible need for repair of the weld in the zone where the penetration is judged insufficient taking into account the no overlap of the welding points.

In the case where the processing means of the control device comprises a means for controlling the power of the welding means, the signal shown in FIG. 5b is used to control the power of the welding means. This power is increased in view of the optical signal corresponding to the pulses 62 and 66 of the pulse of the laser beam and decreased in view of the optical signal corresponding to the pulse 60 of the laser beam.

Claims (5)

1. Apparatus for the real time checking or inspection of a total penetration weld by the detection of the light intensity present on the back of the welding bead at the time of welding, said apparatus being in particular suitable for checking the back of a joint inaccessible to direct observation, said apparatus incorporating an optoelectronic detection means (20) of said light intensity present on the back of the welding bead at the time of welding, a means (22) for processing the electric signal supplied by said detection means (20) and a signalling means (24), said apparatus being characterized in that the optoelectronic detection means (20) comprises an optoelectronic detector (40) and a single optical fibre (42), said fibre having a first end connected to the optoelectronic detector (20) and a second end (56) cut in cone-shaped manner and which has an aperture angle different from 180° and which is kept stationary in a predetermined facing, but set back position with respect to the back of the welding bead to be formed on the parts to be welded, so as to collect the light intensity present on the back of said welding bead at the time of welding, whilst being out of reach for thermal or mechanical actions resulting from the welding, the aperture angle of the end cone (56) of the optical fibre being chosen so that the field of vision (43) resulting therefrom for said end (56) is able, when the latter is kept stationary in said predetermined position, to cover the entire joint defined by the parts to be welded.
2. Apparatus according to claim 1, characterized in that the aperture angle of the end cone (56) is between 25° and 45°.
3. Apparatus according to either of the claims 1 and 2, characterized in that the optical fibre (42) is at least partly contained in a sheath (48).
4. Apparatus according to claim 3, characterized in that the sheath (48) is made from a ductile material.
5. Apparatus according to any one of the claims 1 to 4, characterized in that the processing means (22) supplies a signal for controlling the intensity of the welding beam as a function of the intensity of the detected optical signal.

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